This invention relates generally to data processing, and more particularly to a method and system for adjusting a threshold control in an analog-to-digital converter.
Communication systems often use analog-to-digital converters to sample analog input signals. The analog signals may contain information that will be processed by the communication system. Typically, the analog-to-digital converter receives the analog signal, samples the analog signal at different times, and generates digital representations of the analog signal at those times. Those digital representations form a digital output signal that represents the analog input signal. A processor or other computing device in the communication system uses the digital output signal to approximate the analog signal.
Many types and styles of analog-to-digital converters have been developed. Low-resolution analog-to-digital converters typically sample an analog input signal and generate digital values having one or several bits. In order to use low-resolution analog-to-digital converters in antennas and other components of the communication system, the analog-to-digital converters typically need to maintain a high level of sensitivity.
One problem with conventional analog-to-digital converters is that they often suffer from the formation of direct current (DC) offset voltages within the converters. These offset voltages interfere with the quantization of an analog input signal. For example, if the offset voltage is greater than the voltage of the analog input signal, the analog-to-digital converter would generate the same digital output value, no matter how the analog input signal varies. The converter is unable to quantize analog signals that fall below the offset voltage, and the communication system cannot process the input signal. As a result, maintaining a high level of sensitivity is often difficult when an offset voltage forms in the analog-to-digital converters.
Another problem with conventional analog-to-digital converters is that the decrease in sensitivity also decreases the range of the converters. As the distance between the analog-to-digital converters and a source of the analog input signal increases, the strength of the analog signal decreases. Because conventional analog-to-digital converters often suffer from the formation of offset voltages, the offset voltage eventually becomes greater than the analog input signal. The converters are unable to quantize analog signals that fall below the offset voltage, which limits the effective range of the converters.
In addition, the decrease in sensitivity reduces the signal-to-noise power ratio of the converters. The signal-to-noise power ratio represents the power of the useful information generated by a converter compared to the power of the noise or undesired signals generated by the converter. Because the formation of offset voltages reduces the sensitivity of the converters, the converters generate more noise or undesired signals in the digital output signals. This reduces the effectiveness of the communication system and typically requires additional equipment in the system to remove the noise from the digital signal.
Reducing or eliminating the offset voltage in an analog-to-digital converter is often difficult because the offset voltage routinely changes. The offset voltage that forms in an analog-to-digital converter may change, for example, over time. The offset voltage may also change based on variations in the temperature of the analog-to-digital converter and variations in the manufacturing process used to fabricate the analog-to-digital converter.
The present invention recognizes a need for an improved method and system for adjusting a threshold control in an analog-to-digital converter. The present invention reduces or eliminates at least some of the shortcomings of prior systems and methods.
In one embodiment of the invention, a method for sampling an input signal includes offsetting the input signal with a reference signal. The reference signal represents an offset voltage in the analog-to-digital converter. The method also includes generating a digital output signal based on the offset input signal. The method further includes adjusting the reference signal based on the digital output signal. In addition, the method includes communicating the adjusted reference signal for further offsetting of the input signal.
In another embodiment of the invention, an analog-to-digital converter for sampling an input signal includes a differential amplifier operable to receive the input signal and a reference signal and to offset the input signal with the reference signal. The reference signal represents an offset voltage in the analog-to-digital converter. The analog-to-digital converter also includes a quantizer coupled to the differential amplifier. The quantizer is operable to receive the offset input signal and to generate a digital output signal based on the offset input signal. In addition, the analog-to-digital converter includes a feedback element coupled to the quantizer and to the differential amplifier. The feedback element is operable to adjust the reference signal based on the digital output signal and to communicate the adjusted reference signal to the differential amplifier.
Numerous technical advantages can be gained through various embodiments of the invention. Various embodiments of the invention may exhibit none, some, or all of the following advantages. For example, in one embodiment of the invention, an analog-to-digital converter is provided that may maintain a high level of sensitivity. For example, the analog-to-digital converter may include a feedback element. The feedback element generates a reference signal based on the digital output signal produced by the converter. Because the input signal sampled by the analog-to-digital converter is usually sinusoidal, the converter should produce an equal or near equal number of high and low digital values over time. If the converter produces more high or more low digital values, an offset voltage may be forming in the analog-to-digital converter. The feedback element adjusts the reference signal, and the analog-to-digital converter uses the adjusted reference signal to generate additional digital values. This allows the analog-to-digital converter to adjust the reference signal and to maintain a high level of sensitivity. The high level of sensitivity also helps to maintain a high level of sensitivity in the entire communication system.
Another advantage of some embodiments of the invention is that the analog-to-digital converter may have a greater range than conventional analog-to-digital converters. By reducing or eliminating the effects of an offset voltage in the converter, the converter may quantize input signals at a greater distance from the source of the input signals. As a result, the effective range of the converters increases.
A further advantage of some embodiments of the invention is that the analog-to-digital converter has an improved signal-to-noise power ratio. The analog-to-digital converter generates more useful information compared to the amount of noise or undesired signals generated by the converter. This improves the effectiveness of the communication system and may reduce or eliminate the need for additional equipment to remove the noise from the digital output signal.
In addition, some embodiments of the invention may be less susceptible to variable offset voltages. For example, offset voltages in conventional converters typically vary over time, temperature, and/or fabrication methods. Some embodiments of the invention may reduce or eliminate the formation of offset voltages under these varying conditions. Because the feedback element adjusts the reference signal based on the output of the analog-to-digital converter, this provides a dynamic solution to the formation of offset voltages.
Other technical advantages are readily apparent to one of skill in the art from the attached figures, description, and claims.